Threats to Reefs

Sedimentation

Removal of natural buffers between land and sea, such as
mangroves, allows the products of terrestrial erosion to wash straight
into marine habitats. The coral animal is able to tolerate acute
episodes of sedimentation by secreting mucus as a defense mechanism,
helping to keep their surfaces clean. Nonetheless, destructive activity
on land allows the continual input of sedimentation. This form of
pollution, in particular the fine silt fraction of the sediment,
directly smothers coral reefs blocking out the sunlight required for
photosynthesis (Loya 1976). The short-term effect can interfere with
corals by lowering growth rates, metabolism and fecundity (Rogers 1990).
Prolonged smothering and burial eventually causes the coral to die. In
addition, the silt covers the hard substrate that was available for
settlement of juvenile corals making recruitment of the propagules
impossible and reducing their reproductive success (Gilmour 1999).

As the name would suggest, marine snow resembles snowflakes
suspended in the oceans' water column. Marine snow is the aggregations
of a variety of suspended material consisting of calcareous algae,
organic detritus, and mucus secreted by plankton, algae, bacteria and
corals. Increased nutrient concentrations in coastal waters enhance
algal growth, which indirectly increases the levels of marine snow
(review by Wolanski et al. 2003). Increased carbon levels resulting from
sewage runoff and mucus secretion* also directly influence marine snow.
Until recently, the significance of marine snow and coral reef health
has been neglected. The adhesive property of marine snow means that it
readily attaches itself to suspended sediment (fine clay) from coastal
runoff resulting in it becoming negatively buoyant. This muddy marine
snow is detrimental and even lethal to coral reefs as it settles on the
reef smothering it (Fabricius and Wolanski 2000). Rich in carbohydrates,
marine snow is a source of energy available to microbes and therefore
affecting microbial activity (Kline et al. 2005) including those
associated with the coral tissues. The coral-associated bacterial
populations dramatically increase with increased carbon availability,
which has been seen to be lethal to the coral (Kline et al. 2005). The
following footage shows marine snow in high levels in a severely
impacted coral reef in Borneo, Malaysia.* See video on Mucus Production in Coral Reef Dynamics

The declining health of coral reefs is associated with a
phase-shift from predominantly coral to macro-algal dominated reefs
(Done 1992). Top-down control by herbivores and bottom-up processes such
as eutrophication are critical factors that affect the level of algae
on coral reefs (McCook et al. 2001, Lirman 2001). As the algae encroach
over the coral reef, the activity of coral associated microbial
communities increase dramatically at the boundary between the invasive
algae and coral tissues. Oxygen levels are found to be low around coral
polyps adjacent to the invasive algae due to increased microbial
activity, while coral polyps distanced from the algae have oxygen levels
similar to healthy reefs (Smith et a. 2005). The deficiency in the
levels of oxygen reaching tissues is analogous to suffocation of the
coral animal. This results in a positive feedback loop with both the
algae and coral microbes invading the corals tissues. The eventual
outcome is an ecological shift from a healthy coral dominated reef to an
algal dominated system.

Globally, marine fisheries are under increasing threat due to
unprecedented levels of exploitation. Coral reef areas are heavily
targeted due to increasing human populations. "No-take" marine reserves
are a form of resource management that regulates human activity in order
to conserve and protect local marine habitats. Target species are
expected to increase in abundance and biomass inside the no-take
reserves and eventually influence surrounding areas that are fished
(Russ 2002). For example, the density and biomass of the highly target
coral trout, Plectropomus spp, was found to be significantly higher in
the protected "no-take" zones than in fished zones (Williamson et al.
2004). To be an effective management tool, no-take reserves must have
the compliance of local fishing communities and regulation enforcement.
Unfortunately, most coral reefs reside in some of the poorest regions in
the world where proper infrastructure and enforcement do not exist. The
following footage shows an artisan fisherman searching for lobster in
an area protected as a "no-take" reserve demonstrating how ineffective
this form of resource management is without proper enforcement and
compliance of local communities.

One of the most dominant predators of Pacific corals is the
crown-of-thorns seastar, Acanthaster planci. A. planci is considered a
generalist coral predator with a preference for branching species such
as the acroporids (Carpenter, 2004). The seastar feeds on the coral by
everting its stomach over the live coral and secreting a protease enzyme
that breaks down the coral tissue (Birkeland and Lucas, 1990). Coral
consumption is slow, requiring a leisurely 4-6 hours to digest a coral
branch or a small colony. It is estimated that a single seastar can
consume 5-6 m2 of live coral per year (Birkeland 1989). The immediate
ecological impact of A. planci outbreaks on most reefs is the
modification of community structure and the colonization of algae in
newly available space following coral mortality (Birkeland and Lucas,
1990). The following footage shows slow-moving crown-of-thorns.